libxl: convert an empty tap disk into a qdisk

libxl: convert an empty tap disk into a qdisk

I'm not sure that empty disks which are is_cdrom are especially valid,
or that a cdrom can ever be handled by tapdisk anyway but try to do
something sane since it seems that xl's parse_disk_config() routine
could potentially generate such a configuration (although whether from
a valid input string or not I'm not sure).

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>